This only works for Nook Color?
My Nook Classic isn't recognizing my card at all.
Can someone point me towards more information about this?
Thanks in advance.

I was able to reclaim the lost space by using a partition manager. (In Windows) EASEUS Partition Master has a home version for free that should do the trick. With it, you can delete all the partitions on the SD card and re-format. However, caution is strongly advised. Operating on the wrong storage device can have dire consequences and for most intents and purposes there is no easy undo. It may be easier to just buy a new card if you're not tech-savvy.